Suggestion: Expansion of context passed to `BuildResources`

[TheRSP]

Currently every IBuilder defines a BuildResources method which takes in a single argument which is the location. I propose that this be expanded to a context object which could include the location and other bits of information. In particular I am interested in getting the name of the surrounding resourceGroup.

Example implementation is shown here. This would require a small change to every builder which currently exists but allows future expansions of this context without requiring changes to every builder.

+type rec BuilderContext = 
+  { Location : Location // Target location to store deployment data
+    ResourceGroup : string // The name of the resource group being deployed to 
+    ParentContext : BuilderContext option } // The BuilderContext for the parent deployment where this is in a nested deployment

 type IBuilder =
-    abstract member BuildResources : Location -> IArmResource list
+    abstract member BuildResources : BuildContext -> IArmResource list
     abstract member ResourceId : ResourceId

 type ExistingBuilder
   interface IBuilder with
-    this.BuildResources location =
+    this.BuildResources context =
+      let location = context.Location
       // ... existing BuildResources code

In future, it may be desirable to add deployment scope or other properties into this context.

Why do I want this?

I'm working on a fix for #846 and have found I need to know the name of the resource group so that I can reference resources being deployed from a nested resource group. While I can get around it this time, this will be at least the second time that having the resourceGroup name available would be useful.

[TheRSP]

Further to this, it would seem sensible to also expand the context given to IPostDeploy.Run - it would be useful to have access to the outputs of the ARM deployment in this method which would allow for access to calculated properties like the id of newly-created resources.

[TheRSP]

To make this a non-breaking change, a new interface IAdvancedBuilder could be introduced which would take the new context object. In this situation, an instance of IBuilder could easily be converted to an IAdvancedBuilder:

type IBuilder with
  member this.ToAdvancedBuilder() =
    { new IAdvancedBuilder with
        member _.BuildResources context = this.BuildResources context.Location }

[TheRSP]

@ninjarobot @isaacabraham What do you think about this idea? Benefits:

• We can get rid of the ARM expressions used to name resourceGroups in webApp hostnameBindings/certificates
• We can give better naming for nested deployments (currently, if using Farmer heavily, investigating failed deployments can be hard because the deployment names are reused too frequently)

[TheRSP]

@isaacabraham @ninjarobot Any comments? If you're happy with the proposal I can work on a PR to implement this but I don't want to start unless you agree with the concept.